Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/request/lock/LockHandler.java

@@ -37,7 +37,7 @@ public interface LockHandler extends Serializable {
    *
    * @return true if all locks were acquired and the callable was run; false otherwise.
    */
-  public boolean executeWithLocks(
+  boolean executeWithLocks(
       final Callable<Void> callable,
       @Nullable String tld,
       Duration leaseLength,